Giant planets, terrestrial planets and major satellites

    05-May-2020
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Dr HH Mate
The giant planets are Jupiter, Saturn, Uranus and Neptune. Let me explain about Jupiter and Saturn, Uranus and Neptune, which are the giant planets of the solar systems.
Jupiter : Jupiter, the most massive planet in the solar system, is only about 1/1000 of the mass of the sun. It comes closest in composition to that of the sun itself. If the composition of Jupiter truly matches that of the sun, then it would contain in its total mass the equivalent of about one earth mass of rocky material. However, the best attempts to construct models of the interior of Jupiter indicates the amount of material heavier than hydrogen and helium is significant in excess of that which would be expected for the solar composition. There are probably something like 10  to 20 earth masses of rock and ice in the interior of Jupiter, which is an enrichment of a factor of three to six over the solar composition if the ice-to-rock ratio in the interior of Jupiter is the solar ratio, which is not known. Even this enhanced amount of material amounts to only a few percent of the total mass of Jupiter. The considerable uncertainty in the amount of heavy element enrichment in the Jovian interior results from the uncertainties in the extrapolation of the properties of hydrogen and helium to very high pressures and temperatures such as those found in the interior of Jupiter. It is not even clear whether these heavier materials have settled to the centre of Jupiter, or whether they are suspended in the atmosphere which is being continually mixed throughout the different interior levels of Jupiter due to convective motions.
One of the interesting properties of hydrogen at higher pressures is its tendency to form a conducting metal, metallic hydrogen. Because hydrogen is a simple substance, the physical calculations that lead to the expected transformation from molecular to metallic hydrogen are reasonably certain, but the precise pressure at which this transformation takes place is still quite uncertain. It appears to be somewhat in excess of 106  atm  or  1011 Pascal’s. Most of the mass of Jupiter exists at a pressure considerably in excess of this amount, so that metallic hydrogen is anticipated to form a substantial portion of the interior mass of the planet.
Saturn : Saturn has about only one third of the mass of Jupiter, but nevertheless it also is predominantly composed of hydrogen and helium, and in this case it is definitely clearer that there are heavier elements in excess of solar composition within the interior of Saturn. Again, it is not known whether these heavy elements maintain the solar composition ratio between the ices and rocky materials, and the precise amount of enrichment is therefore uncertain, depending upon this ratio. However, the total amount of heavy materials in the interior of Saturn is comparable to the excess amount in Jupiter.
Heat flow and helium segregation : Attempts have been made to construct evolutionary sequence of models of Jupiter and Saturn which would follow the changes in structure that take place as the planets cool off after their formation. The research has suggested that Jupiter should still be radiating away its interior heat of formation at about the rate which is actually observed as an excess heat flow from the interior, whereas the amount of primordial heat still emerging from Saturn is expected to be much less than is observed. The explanation of this discrepancy may lie in another interesting property expected for a mixture of helium and hydrogen at higher pressures. Below some temperature which is still quite uncertain, it is expected that helium will collect to form small bubbles within the hydrogen, these bubbles, being heavier, will then sink through the hydrogen toward the center of the planet. Not only does this lead to a greater mass concentration toward the center of the planet, but it also releases additional gravitational potential energy, thereby enhancing the heat flow from the interior.
It has been suggested that the interior of Jupiter is still sufficiently hot to have prevented this segregation of helium from hydrogen, whereas the interior of Saturn is sufficiently cooler so that a significant amount of such segregation has and is continuing to occur, thus leading to the observed heat outflow from Saturn.
Uranus and Neptune : Uranus and Neptune are quite similar planets, being 14.5 and 17.2 times the mass of the Earth, respectively. Approximately three quarters of this mass is expected, on the basis of model building, to consist of materials heavier than hydrogen and helium. The precise numbers will depend upon whether these materials are in the solar ratio of ice to rock, which is not known. If one assumes this ratio to be valid, then each of the planets contains approximately four earth masses of rock and approximately twice that much in the form of ices. The remaining hydrogen and helium form a very deep atmosphere.
Physical Compositions : Nowhere in the interiors of the giant planets can anything resembling a solid surface be expected. The temperature in the interiors are very uncertain and be estimated only as the result of model construction, but they tend to be thousands to tens of thousands of degrees celcius. The pressures range upto the order of 107 atm and higher. Under these circumstances all material, behave like fluids. There may be a certain amount of compositional stratification, with denser fluids underlying lighter ones.
This issue of stratification is significant in connection with one of the interesting properties of the interior, the transport of gravitational potential energy released in the deep interior to the surface. The thermal conductivity within the interiors of these planets appears to be much too small to do this job efficiently, even in regions of metallic hydrogen. Conduction may be required to transport heat from a layer of one composition to a neighbouring layer of different composition. But within a layer of any given composition, the transport of heat appears to require convection. Convection consists of an irregular pattern of overturning motions within a liquid, similar to that which occurs when one boils water within a pot. It has been argued on this basis that the interiors of the giant planets are primarily engaged in convective motions which transport heat outward.         
Terrestrial Planets : The terrestrial planets include Mercury, Venus, Earth and Mars. The Earth’s moon may also be considered as terrestrial planet.
Earth : The phototype for the terrestrial planets, and the one about which the most is known, is the earth. The Earth consists of a thin upper crust composed of rocks of relatively low density and low melting points, overlying a much thicker mantle composed predominantly of metallic silicates and oxides, which in turn overlies a substantial core, which is composed of much denser materials, believed predominantly to be iron with other elements, either alloyed or in solution. Most of the core is liquid, but there a smaller inner core which appears once again to be solid, and which probably has some compositional differences relative to the outer core.
On the scale of volatility, the Earth is a very refractory place. Most of the materials in its composition condense at quite high temperatures in a gas of solar composition, usually considerably in excess of 2200oF  or 1200oC. Under such circumstances, most of the iron is expected to be metallic, and since metallic iron is so much heavier than other typical rocky material, such as magnesium silicates, it is natural for the metallic iron to collect at the center of the planet. The detailed seismic evidence indicates that the cone of the Earth is not pure iron, but also has some admixture of oxygen, silicon and sulphur. Several percent of the core must also be nickel, which has properties very similar to that of iron.
The overlying mantle is composed of the oxides and silicates of the metals which are more abundant in nature. Many phase changes take place as such material is subjected to increasing pressure, and some of the increasing density with depth in the Earth’s mantle is due to such phase changes.
Among the many different mineral phases which are present within the Earth, there is a natural sorting process for those minerals which combine a relatively low melting point with low density. Such minerals melt easily and tend to find their way to the surface of the Earth through such cracks or pores as become available. In this way the crust of the Earth is formed predominantly of such materials through tectonic activity.
One of the major revolutions in thinking in the earth sciences has come with the realization that the Earth is a very dynamic place. The position of the earth’s pole has changed dramatically in location with respect to the surface throughout the history of the Earth, and the land masses themselves have drifted about from one part of the surface to another. This continental drift is rendered somewhat easier by the relatively large mass of the Earth and hence the fairly rapid rate with which the temperature increases into the Earth’s interior, thereby weakening the materials and allowing them to deform and flow more easily.
Venus :  The next most massive planet within the inner solar system is Venus, which has slightly more than four-fifths of the mass of the Earth. Venus has a very thick atmosphere, and the temperature at its surface is very much larger than a typical of the Earth’s surface.
The conditions make it very difficult to land spacecraft which can operate for appreciable lengths of time such as would be acquired to obtain seismic signals from the interior of the planet. On these grounds it can only be conjectured that the interior of the planet is probably much like that of the Earth, with a core, a mantle, and a crust.
The pioneer Venus orbiter radar altimeter has found some major structural features on the surface of the planet, suggestive of extensive tectonic activity, but also, to the extent that some of the features are correctly determined to be large craters, indicating that surface weathering processes take place very slowly. The extent to which the crust of Venus is subject to extensive continental drift motions is quite unknown.                            to be continued...
(The author is researcher, educationist, sociologist and author of books. He can be reached at [email protected])